U0101

Lost Communication With TCM

Communication network Generic OBD-II High

What it means

U0101 means the ECM has stopped receiving data from the transmission control module (TCM) over the vehicle's communication (CAN) bus. Without TCM data the transmission usually drops into a fail-safe "limp" mode — stuck in one gear with harsh or no shifting — and warning lights come on. The engine itself normally still starts and runs. Damaged bus wiring, bad grounds, or a failed TCM are the usual causes.

High

Can I keep driving? Limited — the transmission may drop into limp mode (one gear, harsh shifts); drive gently to a shop and avoid highway speeds.

Real-world scan data

Seen in

2,178

scans in the past year

Found in

0.3%

of scans

0.6%

of scans that found faults

Appears alongside · share of U0101 scans

U0100in 33.1% of scans

U0402in 27.4% of scans

U0001in 25.3% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light plus a transmission warning light
Transmission stuck in a single fail-safe gear
Harsh shifting, or no shifting at all
Speedometer or gauge dropouts
A scan tool can't communicate with the TCM

Common causes

Most likely first

Damaged or corroded CAN bus wiring or connectors at the TCMLikely
Poor TCM power or ground, or low battery voltageCommon
Failed transmission control module (TCM)Common
A bus fault dragging down communicationLess often
Rarely, a software or ECM issueLess often

How to diagnose & fix

Work through it in order

Check battery voltage and the TCM's power and ground connections first.
Scan all modules and note which ones are unreachable to narrow it down.
Inspect the CAN bus wiring and connectors at the TCM for damage or corrosion.
Back-probe the bus circuits for opens or shorts.
Repair the wiring, or reprogram/replace the TCM; clear and re-test.

FAQ

Quick answers

Is it safe to drive with U0101?+

Only briefly and gently. With no TCM data the transmission usually locks into a fail-safe gear, so avoid highway speeds and get it to a shop. The engine itself normally still runs.

Can I just clear U0101?+

It returns as soon as communication drops again. The wiring, connection, or TCM behind the lost link has to be repaired.

Read & clear U0101 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX